Interconnection of electronic equipment located at a premises is commonly performed by coaxial cable meeting rigorous standards. Shielded coax cable allows interconnection in a local area network (LAN) covering up to 300 linear feet distance. This is because external radio frequency (RF) energy is not induced in shielded coax cable and because crosstalk energy between parallel cables is suppressed by the use of such cable.
Installing coax cable in a premises is costly and disruptive to the work being performed. In many cases, holes must be made in walls and/or floors and, possibly, an entire subflooring must be installed, to permit routing of coax cable. Once a particular cabling arrangement is established, it can be changed only with difficulty.
Most buildings have an extensive telephone wiring system already in place. It would be highly desirable to use these twisted-pair (TP), unshielded, wires to interconnect electronic equipment, such as computers, workstations, data entry terminals, and the like. Even in buildings being planned, it would be desirable to use ordinary TP telephone wires rather than coax cable because of the formers' ease of installation and lower cost per foot.
But these unshielded TP wires act as an antenna and are susceptible to external radio frequency (RF) energy. This RF noise can swamp a signal carried by the TP wires and thus they provide very poor signal-to-noise (S/N) ratios. Particularly, when long distances such as 300 feet are involved, as would be typical of a LAN.
S/N ratios can be improved by using very high signal transmission power levels, but FCC regulations specify maximum tansmission levels and high power tansmission is costly. A better solution is to provide a totally differential balanced receiver and to filter at transmission to fall within the FCC regulations, and filter at receiver to 40-100 megahertz (MHz) which then eliminates bulk of external RF noise susceptibility.
Known in the prior art are linear bandpass filters which are used to attenuate all signals outside of the 40-100 MHz band of frequencies. Such filters are expensive and do not provide the impulse rejection needed to avoid false activation of a receiver employed a LAN. Because the LAN to be employed with the instant invention is to be interconnected by TP unshielded wires, such impulse rejection is especially important.
What is required is a fast-attack discriminating circuit which provides both bandpass filtering and "squelching" of impulses.